copy surface geometry and put it somwere outside of the feeld of view.
make it plain black

extrude a square along the animation spline. make it pure white. width of the car
set a camera from above and use the resulting picture (camerashader) as an alpha to reveal a predefined bumpmap on the original surface.
if you make two setups, one for the front axis of the car and one for the rear you maybe get good results without a lot of geometry.

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